The use of fiber optic proximity probes or fotonic sensors utilizing bifurcated fiber optic bundles and sensors and detectors to determine the distance to a target are well known. Such devices utilize a light beam transmitted from a light source by way of a bundle of light conducting optical fibers (transmit fibers) from a light source to a target. The light is reflected off the target and returned along other optical fibers (receive fibers) in the bundle back to a light intensity sensor. The intensity of the returned light is a function of the distance between the proximity probe tip and the target, so the output of the light sensor can be a very precise measure of that distance, or more importantly changes in that distance. Such a system is shown and described in U.S. Pat. No. 3,940,608 in which an extender lens is utilized.
However, it has been found that due to variations in the operating parameters, particularly the reflectivity of the target, a manual adjustment was necessary i.e., increasing or decreasing the source intensity or distance of the probe from the target, in order to maintain the accuracy of the distance reading once reflectivity had changed. While in certain applications, such manual readjustment was acceptable, in other situations where perhaps the probe is inaccessible or required to be fixedly mounted, such manual adjustment becomes impossible.
Accordingly, there arose a need to provide for automatically compensating for a change in reflectivity. While there have been many attempts to provide for such a feature, there still remains a need for a single yet effective means of compensating for reflectivity changes.